An escalator typically has five different zones when viewed in elevation. These are: the entrance zone, which is horizontal; the entrance transition zone, which has a gradually increasing slope angle; the medial inclined zone, which has a fixed slope angle; the exit transition zone, which has a gradually decreasing slope angle; and the exit zone, which is horizontal. The step path is thus arcuate in the transition zones, and linear in the horizontal and inclined zones, in elevation. Likewise, the upper and lower margins or edges of the balustrade have linear and arcuate components. A curved escalator has a slightly more complicated geometry due to its curvature in plan, however, the elevational projections of the curved escalator are generally the same as the linear escalator. Generally, the handrail of the escalator will move along a guide rail which is mounted on the upper edge of the balustrade. Thus, the handrail will have a path of travel, in elevation, that follows the elevational projection of the upper edge of the balustrade.
When determining the radii and angles of incline of the path of travel of the escalator steps, a convenient reference point in elevation is the nose of the steps. The "nose" of the step is the line where the step tread and riser meet on the step, which is, in elevation, a point. These serial points thus provide a good reference for the serial steps which can be placed on the desired line along which the steps should move from entrance to exit. It is highly desirable that an escalator passenger not be able to detect any apparent vertical changes in the position of the handrail relative to him or herself during the ride from entrance to exit. This means that there should not be a perceptible rise or fall of the handrail sensed by the passenger during the ride. The apparent solution to the problem would seem to be to have the upper edge of the balustrade track the path followed by the nose of the steps, however, when this approach is followed, the handrail will perceptively rise when the medial inclined portion of the escalator is reached since the plane of the treads on which the passengers stand lies below the line connecting serial step nose points in the incline. Thus, the upper edge of the balustrade should not track the line which determines the path of travel of the steps or step chains.